Reduction of nitrogen oxide (NO.sub.x) emissions from exhaust or flue gases, before they are released into the atmosphere, has been a prolific topic of discussion in the field of environmental aspects of energy production by combustion of fuel material. Because NO.sub.x emissions are related to various environmental problems, the minimizing of NO.sub.x release from combustion systems is an ongoing concern.
It is evident that nitrogen oxide emissions result from any combustion reaction where air is present and/or the fuel used contains nitrogen. Fluidized bed combustion of fuel is a well known practice has been found to be beneficial in reducing nitrogen oxide emissions due to its relatively low operating temperature. In fluidized bed combustion, air is typically introduced through a plenum, where it is distributed through an air distribution grid. Fuel, fluidizing solids and possible sorbents, such as limestone or dolomite, are fluidized, and they react in the furnace at temperatures normally within the range of about 700-1200.degree. C.
Heat is recovered from hot gases and solid material if present in steam generation boilers by using heat transfer surfaces which are most often located in the combustion chamber, i.e. the furnace, and the convection section downstream the combustion chamber. In the combustion chamber heat transfer surfaces may be located on the peripheral walls, the walls then being e.g. membrane walls, and, especially when large heat transfer area is needed, also within the gas space of the combustion chamber as internal heat transfer surfaces, e.g. as wing wall tube panels or omega tube panels. The wing wall tube panels and omega tube panels are special structures which are designed to withstand harsh conditions prevailing, e.g., in the combustion chamber of a fluidized bed boiler.
Nitrogen oxides are generated during combustion of fuels as a result of thermal fixation of nitrogen in the air and the conversion of fuel nitrogen. The former reaction is favored at high temperatures (above about 950.degree. C.) while the latter is of greater concern at lower temperatures, e.g. those generally found in fluidized bed combustion systems.
U.S. Pat. No. 3,900,554 suggests removal of nitrogen oxides from flue gases, which have exited a conventional furnace, by injecting ammonia (NH.sub.3) into the effluent stream. The main problems in applying the method disclosed in U.S. Pat. No. 3,900,554 are related to ensuring that the reducing agent is sufficiently mixed with the flue gases and that it has a sufficient retention time at an optimum reaction temperature.
European Patent Application No. 0 176 293 suggests the use of NH.sub.3 for NO.sub.x control in circulating fluidized bed reactors (CFB reactors) via ammonia injection into the flue gas stream immediately prior to its entry into the centrifugal separator. By this method an improved mixing of the reducing agent and the flue gases is obtained, but the retention time may still be too short. Moreover, the reaction temperature may be too low, especially at low load conditions.
It has also been suggested to provide longer retention times and higher reaction temperatures by introducing reducing agent through nozzles disposed at various locations on the walls of the furnace itself. This method, however, only allows a rather shallow penetration of the reducing agent into the furnace, and therefore the main gas stream in the central region of the furnace and therefore not efficiently mixed with the reducing agent. Especially in fast fluidized bed reactors, a dense layer of downward flowing solid particles tends to form close to the furnace wall. This layer prevents introduction of gas through the walls. In fluidized bed reactors there is also a potential for injected ammonium-based reducing agents, such as ammonia, to convert into NO.sub.x over the solid particles concentrated along the walls.
U.S. Pat. No. 4,181,705 suggests introduction of ammonia or an ammonia-releasing compound, in fluidized bed combustion systems, into the fluidized bed along with fuel and air used for fluidization and combustion. In this method, however, there may be a risk of too high temperatures and consequently oxidation or dissociation of the ammonia before it comes into contact with nitrogen oxides produced in the combustion process.
U.S. Pat. No. 4,115,515 teaches reduction of NO.sub.x emissions by disposing at different locations within the flue gas path a plurality of separate manifold and tubular injector structures, with spaced apertures or nozzles, for introducing ammonia. The particular location actually used for injection of ammonia has to be determined by measuring the temperature of the gases at the different locations, in order to inject ammonia at a suitable temperature and to prevent dissociation of ammonia at too high temperatures. It may, however, still be necessary to separately insulate or water-cool the manifolds, tubular injectors and nozzles for the same reason. The nozzles are disposed in the injectors so as to direct ammonia counter-currently against the gas path. The whole cross section of the gas path, e.g. the convection section or the outlet of the furnace, has to be covered by nozzles to ensure efficient mixing of ammonia with the gases.
The plurality of means for injecting a reducing agent with many nozzles, traversing the flow path of the hot gases at different locations, brings a plurality of additional obstacles into the gas path, which generally should be avoided, as such obstacles are space consuming, have an influence on the gas flow, and have to be cleaned from time to time.
Heretofore, no simple and durable solution for the location of the means for injecting the reducing agent has been provided. There exists a need especially for finding an improved simple solution for the injection of the reducing agent within the furnace and for providing sufficient cooling of the reducing agent when injected.